Another idea, the Walker called Mac "The Watcher," Mac may be "Out," but I wonder if he is still "watching." If I remember correctly, Mac only came to Chicago a few years before Harry, and Mac has appeared in almost every book. What if Mac is Watching Harry. Harry would not tolerate someone he saw or suspected was an enemy (white council or winter court) to watch him, and he would react badly (because it's Harry's nature). Knowing this, interested parties (white council, winter court, others?) planted someone in Chicago who Harry would not suspect as "watching" him. While Mac might not see Harry every day, he sees him enough to check his mental state and general wellbeing. Also, Mac knows a lot of people in town, and he can use them to learn more info. Also, people talk to and in front of bartenders like they aren't even there. This is another source of info for Mac.